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Re: [psk31] PSK-31 Satellite Donwlink Opportunity? (fwd)



---------- Forwarded message ----------
Date: Mon, 26 Feb 2001 18:20:51 +0000
From: Peter Martinez <Peter.Martinez@btinternet.com>
To: psk31@bipt106.bi.ehu.es
Subject: Re: [psk31] PSK-31 Satellite Donwlink Opportunity?

>From Peter G3PLX

The PSK31 satellite proposal raised by Bob is an interesting idea if
it could work to cancel Doppler shift, but I don't think his proposal
for an inverting in-band transponder will do this.

It's true that if I was to transmit to a moving satellite fitted with
a frequency-inverting transponder and receive my own signal coming
back to me on a nearby frequency, the Doppler shift would cancel out,
but consider what happens if the satellite is moving towards me but
away from the station I am working. In this case the Doppler shifts
don't cancel but add together. With this reasoning and if you suppose
you are equally likely to want to work stations everywhere within the
footprint, the inverting in-band transponder is just as bad as the
non-inverting type.

There's another problem with using a transponder which is operating
within one band as Bob is proposing, namely that the vast majority of
amateurs will be unable to "listen-through" on the same band as they
are transmitting, and so will not be able to "net" on the same
frequency as the other station. This would make the task of finding
replies in a crowded passband a very difficult process. Even if you
tried to calculate the transmit frequency to use (knowing the
satellite conversion oscillator frequency), you run into problems if
it's an inverting in-band transponder, because the conversion
frequency is the sum of the uplink and downlink frequencies and THIS
frequency is Doppler shifted too and will therefore appear different
to every station.

There is perhaps a case for a non-inverting in-band transponder to
overcome this. The conversion frequency will be low (1.4 MHz) and
suffer negligible Doppler. In the case of a PSK31 transponder, all
stations would set their transmitter and receiver  frequencies split
by precisely 1.4 MHz (or whatever the actual conversion frequency of
the satellite was) and work with the PSK31 software running
co-channel simplex. There would still be the basic Doppler problem,
but at least if you call a station on the same audio frequency as you
receive the end of his call, he will hear the start of your reply on
the same audio frequency as he was transmitting.

Could someone copy this message to the amsat reflector - I am not a
subscriber.

73
Peter
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